Wake-up Attacks: Wake-up attack system is identical to MK9. The first 8 frames (8 frames is a rough estimate, not literal so keep that in mind) are invinsible for your special move, after that you are vulnerable. There are a couple things that are different from MK9 to Injustice though.
In MK9, you can do an enhanced special move which increases the amount of frames of invincibility for your wake-up attack. This is not the case in Injustice since most of the 'enhanced' versions of special moves only come out after the special move hits.
Another note is that a lot of character's special moves last longer than 8 frames in Injustice. Not everyone has a Lex Corp Charge (b,f+2) which is 6 frames so the entire duraction of the move will be invincible. So a lot of you will notice your wake-up attacks getting 'stuffed' because you lost invincibility after the 8th frame. If you find your opponent is purposely delaying their attack to hit you out of your special move when your invincibility wears off, there's a couple things you can do.
a) Jump: Since their attack isn't 'meaty', meaning they aren't hitting you on the first available frame you can get hit when your getting up, you will have enough frames to get a jump off. Granted you might still get hit, but you will be airbourne and chances are they won't be able to follow up and you can roll away once you hit the ground.
b) b+3 or f+3 mb armor: b+3 and f+3 when activated with meter will gain armor after the 8th frame. So if your opponent is keeps trying to suff your wake-up with delayed attacks, do a b+3 or f+3 mb as soon as you get up to armor out and get a good punish.
Once your opponent realizes they can no longer do delayed attacks to stuff wake-up attacks, they will start blowing you up with meaty attacks as you get up, which opens up you doing your actual wake-up attacks with invincibility on them again.
